Use of 5-aminolevulinic acid for reducing nitrate nitrogen content in plants

Abstract: Methods for reducing nitrate nitrogen and oxalic acid contents in plants, each of which comprises applying 5-aminolevulinic acid, a salt thereof or a mixture thereof. Nitrate nitrogen and oxalic acid contents in plants having high nitrate nitrogen and oxalic acid contents can be reduced effectively and easily.

Description: FIELD OF THE INVENTION This invention relates to the use of 5-aminolevulinic acid, a salt thereof or a mixture thereof for reducing in plants the content of nitrate nitrogen which causes the formation of toxic nitrosamine. BACKGROUND OF THE INVENTION It is known that nitrate nitrogen present in food is partly reduced into nitrous acid by enteric bacteria in the living body and the nitrous acid thus formed results in the formation of mutagenic nitrosamine through its binding with secondary amine present in other food. Nitrosamine is toxic for human health because it is considered to trigger the formation of cancers, tumors and the like. In consequence, great concern has been directed toward the reduction of nitrate nitrogen content in food. In addition, since the incorporation of nitrate nitrogen into the human body originates from food, particularly from vegetables, it is most important to reduce the nitrate nitrogen content in vegetables. On the other hand, the problems caused by nitrate nitrogen are common not only in human provisions but also in forage crops for domestic animals. Because of this, a standard nitrate nitrogen content in feed has been established, and reduction of the nitrate nitrogen level in feed therefore is an essential subject to be accomplished. In view of the above, cultivation methods, especially fertilizer application methods, have been examined for the reduction of nitrate nitrogen content in plants. In these methods, the amount of fertilizer applied is reduced or a slow-acting fertilizer is used. However, such countermeasures limit the quantity and quality of fertilizers and therefore cause problems of slow growth and small yield of plants in comparison with commonly used cultivation methods. Also, a method in which the nitrate nitrogen content in plants is reduced by the use of a fertilizer containing a nitrification inhibiting agent has been developed and partly put into practical use, but such a method resulted in high cost and its effect in reducing the nitrate nitrogen content was not sufficient. SUMMARY OF THE INVENTION In view of the above, it is therefore an object of the present invention to reduce the content of nitrate nitrogen in plants. Taking the aforementioned problems involved in the prior art into consideration, the inventors of the present invention have conducted intensive studies with the aim of finding a chemical agent which can reduce the content of nitrate nitrogen in plants and found unexpectedly that 5-aminolevulinic acid, its salt or a mixture thereof functions to reduce the content of nitrate nitrogen in plants to a satisfactory level. The present invention has been accomplished on the basis of this finding. Accordingly, the present invention provides the use of 5-aminolevulinic acid, its salt or a mixture thereof for reducing nitrate nitrogen content in plants. In this connection, the present inventors already have proposed a method for applying 5-aminolevulinic acid or a salt thereof to plants by which growth, rooting, photosynthetic activity and the like of plants is promoted (U.S. Patent 5,298,482). However, the effect of reducing the content of nitrate nitrogen in plants as disclosed herein is a newly found effect which cannot be expected from the previously proposed growth-promoting and the like effects. S.N. Mishra and H.S. Srivastava, Experientia 39 (1983) 1118-1120, refer to the stimulation of nitrate reductase activity by delta amino levulinic acid in excised maize leaves. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 5-Aminolevulinic acid, a salt thereof or a mixture thereof to be used as the active ingredient of the present invention is a known natural compound which can be produced by any known method such as chemical synthesis, microbial production, enzymatic synthesis, extraction from natural sources or the like. In the case of the microbial production for example, the fermentation product may be used as it is without employing separation and purification steps, provided that the product does not contain substances which spoil the effect of the present invention. Examples of the salt of 5-aminolevulinic acid include acid addition salts such as the hydrochloride, phosphate, nitrate, sulfate, acetate, propionate, butyrate, valerate, citrate, fumarate, maleate, malate and the like salts and metal salts such as the sodium, potassium, calcium and the like salts. In order to effect a reduction in the content of nitrate nitrogen in plants by using 5-aminolevulinic acid, a salt thereof or a mixture thereof, any treating method such as foliar application, soil treatment or the like may be used, provided that 5-aminolevulinic acid, a salt thereof or a mixture thereof is absorbed by plants. Any treating or application method can be used as long as the method produces the plant with the ability to absorb the 5-aminolevulinic acid, salt thereof or a mixture thereof. In the case of hydroponics and the like, 5-aminolevulinic acid, a salt thereof or a mixture thereof may be used in the form of an aqueous solution to achieve root absorption. When 5-aminolevulinic acid, a salt thereof or a mixture thereof is used in foliar application, it can contain preferably from 1 to 1,000 ppm, more preferably from 5 to 500 ppm, of 5-aminolevulinic acid, and it can be applied in an amount of preferably from 10 to 3,000 1, more preferably from 20 to 1,000 1, per 1,000 m 2 . When 5-aminolevulinic acid, a salt thereof or a mixture thereof is applied to a plant whose leaves can hardly retain the active ingredient, it is preferable to use a spreader together with 5-aminolevulinic acid. The type and amount of the spreader are not particularly limited and can be determined appropriately in a conventional manner. When 5-aminolevulinic acid, a salt thereof or a mixture thereof is used in soil treatment, it can be applied to soil in an amount of preferably from 0.5 to 800 g, more preferably from 1 to 300 g, of 5-aminolevulinic acid per 1,000 m 2 . When 5-aminolevulinic acid, a salt thereof or a mixture thereof is to be absorbed through roots of plants in the form of an aqueous solution such as the case of hydroponics, it contains preferably from 0.001 to 50 ppm, more preferably from 0.01 to 20 ppm, of 5-aminolevulinic acid. 5-Aminolevulinic acid, a salt thereof or a mixture thereof exerts its effect satisfactorily by a single operation of any application process described above, but the effect is improved further by repeating the process several times. Though not particularly limited, the application may be carried out preferably at an early stage of plant growth. 5-Aminolevulinic acid, a salt thereof or a mixture thereof may be used in the form of a mixture with other agricultural chemicals, fertilizers and the like, provided that they do not adversely affect the present invention. With regard to the use of 5-aminolevulinic acid, a salt thereof or a mixture thereof for reducing the nitrate nitrogen content in plants, according to the invention plants to be treated are not particularly limited, but its effect is markedly evident when a plant having a high nitrate nitrogen content, preferably 0.5% by weight or more, more preferably 1% by weight or more, of nitrate nitrogen of the total nitrogen content in the plant, is treated. Illustrative examples of such plants include vegetables such as spinach, Chinese cabbage, cabbage, lettuce and the like, pasture grasses such as maize, clover and the like and weeds such as a broadleaved plantain, a Japanese knotweed, a wood sorrel, a sorrel and the like. The following examples are provided to further illustrate the present invention. It is to be understood, however, that the examples are given for purpose of illustration only and are not construed as limiting the scope of the present invention. Unless otherwise indicated herein, all parts, percents, ratios and the like are by weight. EXAMPLE 1 On October 20, 11 spinach seeds (Glory, available from Takii Shubyo K.K.) were sowed in a pot of 1/500,000 m 2 filled with field soil. As a basal fertilizer, 1.5 g of a compound fertilizer Sosai No. 3 (N:P 2 O 5 :K 2 O = 15:15:15) was applied to each pot but with no additional dressing. As the seedlings grew under usual tending conditions in a greenhouse, thinning was repeated to select 3 uniform seedlings to be grown further in each pot. On November 11, foliage treatment was carried out with 20 ml per pot of an aqueous solution containing 0, 50, 100 or 300 ppm of 5-aminolevulinic acid hydrochloride (hereinafter referred to as "5-ALA") and 0.1 ml/100 ml of a spreader (Approach (mainly composed of 50% of polyoxyethylenehexytane fatty ester and 20-30% of isopropyl alcohol), manufactured by Kao Corp.). Each treatment was carried out using 6 pots. After further tending under usual conditions in the greenhouse, the plants were harvested on January 18. The nitrate nitrogen content in the plant leaves was measured using HPLC in a conventional manner to calculate the ratio of the nitrate nitrogen in total nitrogen. The results are shown in Table 1 below. 5-ALA Conc. (ppm) Nitrate Nitrogen Content (mg/100 g DW ) Ratio (%) 0 4.83 14.59 50 5.00 9.06 100 4.20 6.71 300 3.57 6.41 As is evident from the results shown in Table 1 above, the nitrate nitrogen content is reduced by the method of the present invention, with especially great reduction in the ratio of nitrate nitrogen to total nitrogen. EXAMPLE 2 On June 11, 5 maize seeds (Yukijirushi Snowdent RM120, available from Yukijirushi K.K.) were sowed in a pot of 1/500,000 m 2 filled with field soil. As basal fertilizers, 6 g of a compound fertilizer (N:P 2 O 5 :K 2 O = 10:10:10) and 5 g of Magporon (mainly composed of dolomite (CaCO 3 ·MgCO 3 )) were applied to each pot but with no additional dressing. As the seedlings grew under usual tending conditions in a greenhouse, thinning was repeated to select 2 uniform seedlings to be grown further in each pot. On August 1, foliage treatment was carried out with 20 ml per pot of an aqueous solution containing 0, 50, 100 or 300 ppm of 5-ALA and 0.1 ml/100 ml of a spreader (Approach, manufactured by Kao Corp.). Each treatment was carried out using 6 pots. After further tending under usual conditions in the greenhouse, the plants were harvested on August 14, air-dried and then pulverized to measure the nitrate nitrogen content using HPLC in a conventional manner and to calculate the ratio of nitrate nitrogen in the total nitrogen. The results obtained are shown in Table 2 below. 5-ALA Conc. (ppm) Nitrate Nitrogen Content (mg/100 g DW ) Ratio (%) 0 3.20 14.18 50 2.35 7.75 100 1.60 6.66 300 3.25 8.41 As is evident from the results shown in Table 2 above, the nitrate nitrogen content is reduced by the method of the present invention, with especially great reduction in the ratio of nitrate nitrogen to total nitrogen.

Claims: The use of 5-aminolevulinic acid, a salt thereof or a mixture thereof for reducing nitrate nitrogen content in a plant. The use of claim 1, characterized in that the nitrate nitrogen content in the plant to be treated is 0,5 % or more by weight of the total nitrogen in said plant. The use of claim 1 or 2, characterized in that said salt of 5-aminolevulinic acid ist at least one salt selected from the group consisting of the hydrochloride, phosphate, nitrate, sulfate, acetate, propionate, butyrate, valerate, citrate, fumarate, maleate, malate, sodium, potassium and calcium salts. The use of any one of claims 1 to 3, characterized in that a spreader is further present.

Inventors: HOTTA YASUSHI, TANAKA TOHRU, YOSHIDA RYUJI, HOTTA, YASUSHI, TANAKA, TOHRU, YOSHIDA, RYUJI, HOTTA, YASUSHI, C/O COSMO RESEARCH INSTITUTE, TANAKA, TOHRU, C/O COSMO RESEARCH INSTITUTE
Applicants: COSMO OIL CO LTD, COSMO RES INST, COSMO OIL COMPANY, LTD, COSMO RESEARCH INSTITUTE
Application year: 1994
Main classification: A01N37
IPC codes: A01N37